Just to clarify, the runegraver uses d8 hit dice. The hit die type affects how many hit points you gain per level and also how many hit points you can heal per day of rest. Saving throws, however, are based on the total number of hit dice (for monsters) or levels (for classed characters), not on the hit die type. In short, a 1st-level character or 1-HD monster has a base SV of 16, and an 8th-level character or 8-HD monster has a base SV of 13, regardless of hit die type.